The purpose of this test is to evaluate the operation of an Antilock Braking System (ABS) inductive wheel speed sensor based upon its output voltage amplitude and frequency.
Access to individual wheel speed sensors may be difficult. However, all wheel speed sensors connect to the ABS control module, which is usually located in the engine bay.
These known good waveforms have the following characteristics:
Wheel speed sensors provide wheel and road speed feedback to ABS and derivative active vehicle safety systems (i.e. stability and traction control etc.).
These systems are designed to provide corrective action (e.g. wheel braking or engine torque limitation) when the vehicle chassis or wheel speeds have exceeded their normal operational tolerances, for example, during conditions of wheel slip, oversteer or understeer etc. Wheel speed sensors are critical to the operation of these systems and, therefore, the safe handling of a vehicle.
Inductive wheel speed sensors encase a wire coil around a magnetically polarised core and operate in tandem with a pulse wheel mounted on a component rotating at wheel speed, for example, a driveshaft or hub. As the pulse wheel passes the sensor, the sensor’s magnetic field is disturbed, which induces an oscillating voltage within the wire coil and the circuit connected to it. The quicker the wheel rotates, the greater the magnetic field disturbance, the greater the induced voltage amplitude and the faster the oscillation frequency.
It is not easily possible to distinguish two wire passive (inductive) and active (Hall or magnetoresistive) wheel speed sensors by their external appearance. Diagnostically, this is inconvenient as active wheel speed sensors must never be subjected to resistance tests: this can damage these units, with the only remedy being the acquisition of a new replacement.
Therefore, you must always either consult manufacturer’s data to identify the fitted type prior to diagnostic testing or carry out a speculative PicoScope check to identify the sensor from its output voltage characteristics.
You can check for a positive supply voltage at one of the sensor connector terminals to determine if you have an active sensor type. However, if the supply voltage is missing due to a fault and you then assume the sensor must be passive and perform a resistance check, you can damage a perfectly good active sensor. This will give you two faults.
An ABS control module expects similar (within a given tolerance) oscillation frequencies from all the vehicle’s wheel speed sensors and uses any differences to calculate the timing and scale of its interventions.
If one, or more, wheel speed signals continuously fall outside of normal parameters the control module may turn the ABS function off (along with associated traction and stability systems). A driver warning light will be illuminated but, as with any electrical fault on ABS, normal hydraulic braking is maintained.
Wheel speed sensors and their pulse rings are exposed to the atmosphere and have to operate under conditions of constant vibration and movement. As such, common faults are:
Wheel speed sensor circuits and connectors are also prone to the atmosphere and possible electrical failures, such as open or short circuits or high circuit resistances.
Symptoms of ABS sensor related faults
Selection of component-related Diagnostic Trouble Codes (DTCs):
C0000 - Vehicle Speed Information Circuit Malfunction
C0035 - Left Front Wheel Speed Circuit Malfunction
C0036 - Left Front Wheel Speed Sensor Circuit Range/Performance
C0040 - Right Front Wheel Speed Circuit Malfunction
C0041 - Right Front Wheel Speed Sensor Circuit Range/Performance
C0045 - Left Rear Wheel Speed Circuit Malfunction
C0046 - Left Rear Wheel Speed Sensor Circuit Range/Performance
C0050 - Right Rear Wheel Speed Circuit Malfunction
C0051 - Right Rear Wheel Speed Sensor Circuit Range/Performance
C0221 - Right Front Wheel Speed Sensor Circuit Open
C0222 - Right Front Wheel Speed Signal Missing
C0223 - Right Front Wheel Speed Signal Erratic
C0225 - Left Front Wheel Speed Sensor Circuit Open
C0226 - Left Front Wheel Speed Signal Missing
C0227 - Left Front Wheel Speed Signal Erratic
C0229 - Drop Out of Front Wheel Speed Signals
C0235 - Rear Wheel Speed Signal Circuit Open
C0236 - Rear Wheel Speed Signal Circuit Missing
C0237 - Rear Wheel Speed Signal Erratic
C0238 - Wheel Speed Mismatch
C0245 - Wheel Speed Sensor Frequency Error
C0300 - Rear Speed Sensor Malfunction
C0305 - Front Speed Sensor Malfunction
We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.